A wide variety of gaskets are known for use in sealing applications. Porous expanded polytetrafluoroethylene (ePTFE) is widely used today as a gasket material. As disclosed in U.S. Pat. No. 3,953,566 to Gore, this material has numerous properties making it highly desirable as a gasket. These properties include compressibility, conformability, chemical resistance, high strength, and resistance to creep relaxation and loss of sealing pressure.
In many sealing applications, the gasket is used to seal the junction between flanges, such as between pipes. In such applications, expanded PTFE is a desirable material for the gaskets because the expanded PTFE gasket can be placed between the flanges, and the flanges can then be pressed together with the application of force, such as by tightening of bolts. This application of force compresses the expanded PTFE. As the expanded PTFE is compressed, its initial pore volume is reduced, thus densifying the expanded PTFE. Particularly with metal-to-metal flanges, it is possible to apply sufficient force (or “stress”) to the flanges to fully densify the expanded PTFE. Thus, in at least part of the expanded PTFE gasket, the pore volume is reduced to substantially zero, such that a fluid contained within the pipes is prevented from leaking between the flanges by the densified expanded PTFE gasket, which seals the flanges.
In many applications, particularly when harsh chemicals are used which would readily breakdown metal which could contaminate the chemical which is being transported or housed, it is common to use glass-lined steel, glass, or fiberglass reinforced plastic (“FRP”) piping and vessels. Because this equipment is often used with extremely harsh chemicals, there is great desire to use PTFE gaskets to seal the connecting flanges of this equipment because of the well known extraordinary chemical resistance of PTFE. Unfortunately, non-expanded, non-porous PTFE gaskets may not be conformable enough to effectively seal this type of equipment. In the case of glass-lined steel flanges, although there is a relatively smooth finish, there is often a large amount of unevenness or lack of flatness associated with the flanges. This unevenness or lack of flatness requires the gasket to conform to large variations around the perimeter as well as between the internal and external diameter of the flange in order for an effective seal to be created. It would be desirable to use a conformable expanded PTFE to seal these commonly uneven flanges.
In many applications it is not possible to apply sufficient force to the flanges to create enough gasket stress to sufficiently densify the expanded PTFE gasket to create an effective seal. For example, glass-lined steel piping flanges, glass flanges, or FRP piping flanges may deform, fracture, or break upon the application of a high amount of stress. In these applications, an expanded PTFE gasket may not become leak proof because the maximum stress that can be applied to the flanges without breaking them may not be sufficient to densify the gasket to a non-porous state. Where the expanded PTFE gasket is not sufficiently densified, leakage can occur through the residual porosity within the gasket. In such cases and where corrosive chemicals are being processed, a leak may persist undetected for months or years until the corrosive chemicals eventually leak through the gasket and attack uncoated areas on the outside of a flange resulting in severe damage to the flange. If gone unnoticed for a long enough period of time, the chemical attack on the outside of the flange can result in a catastrophic failure of the gasketed joint.
U.S. Pat. No. 6,485,809, in the name of Minor et al., teaches a low stress to seal gasket construction which provides a substantially air tight, or air impermeable, seal upon the application of a relatively low stress. One embodiment is a multilayer, unitary gasket having at least one inner layer of expanded PTFE disposed between a first substantially air impermeable outer layer and a second substantially air impermeable outer layer, and a substantially air impermeable region bridging the first and second substantially air impermeable layers. Gaskets are stamped or cut from multilayered laminated sheets formed by wrapping layers around a mandrel, and are subjected to compressive treatment to compress a discreet portion forming an air impermeable region. While this patented construction may overcome many challenges in creating a low stress to seal gasket, the size of the gasket that can be produced when cutting from sheet gasketing is limited to the sheet size itself. Also, tooling costs for large size gaskets can be quite expensive and the manufacturing efficiencies of cutting gaskets from sheet stock can be relatively low especially with large diameter gaskets, where much of the sheet is scrapped.
U.S. Pat. No. 5,964,465 to Mills et al. teaches a biaxially expanded PTFE form-in-place type gasket having the advantage of being able to be formed to any size flange without the limitations of gaskets cut from sheet stock. Form-in-place gaskets made in accordance with the teachings of Mills et al., comprised of biaxially expanded PTFE, may have additional advantages offered by the biaxially expanded PTFE such as chemical resistance, dimensional stability, and resistance to creep relaxation. However, as previously noted, in many applications adequate gasket stress cannot be applied to sufficiently densify the ePTFE, therefore, these gaskets cannot effectively seal glass lined steel and FRP flanges.
In PCT publication WO01/27501 A1 to Dove et al., a form-in-place gasket comprising an inner layer of expanded PTFE and substantially air impermeable outer layers that are bridged by a substantially impermeable region is taught. While the substantially air impermeable outer layers and substantially air impermeable region are intended to prevent permeation through the expanded PTFE gasket material, gaskets constructed according to the teachings of WO01/27501 are subject to a number of disadvantages. For example, gaskets having a substantially air impermeable region bridging the first and second substantially air impermeable layers along the inner or outer perimeter of the formed gasket may be susceptible to leakage at the overlap of the two ends of the form-in-place gasket. In glass lined steel flanges, it is common that there is a curvature in the radial direction of the flange across the width of the flange. This results in a stress concentration towards the center of the gasket which also results in a minimal amount of stress being applied to the outer edges of the gasket. As a result, process fluids can penetrate into or out of the gasket through the exposed porosity at the overlap of the two ends. Moreover, where the substantially air impermeable region is located between outer edges of the gasket as shown in FIGS. 11 and 12 of WO01/27501, such gasket constructions are susceptible to fracturing at the overlap of the two ends of the gasket. Even at relatively low bolt loads the stress on the overlap where two impermeable regions are stacked on one another may exceed the fracture point of the ePTFE causing the gasket to rupture at this location. The integrity of the gasket at the point of rupture is lost and a leak path may result causing the gasket to fail.
In U.S. Patent Publication No. 2003/0003290 A1 to Hisano et al., a sealing material in the form of a tape is taught having laminated layers of porous expanded PTFE slit into strips having a height greater than the width. In use the laminated end faces on the long side of the strips are oriented to be in contact with the tightening surface. A plurality of the laminated strips may be joined together on the laminated surfaces with tetrafluoroethylene-hexafluoropropylene copolymer or tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer film. It is further taught that at least one layer may be interposed within the laminate for preventing fluid penetration. The longitudinal beginning and end of the tape are joined to form a gasket, and the layers of expanded PTFE and the layer for preventing fluid penetration are vertically oriented when the gasket is installed on a flange surface. The layers for preventing fluid penetration are intended to prevent leakage in the radial direction through the porous ePTFE, thus providing a low stress to seal gasket by eliminating the need to apply sufficient stress to densify the porous ePTFE. For gaskets made according to this method the ePTFE layers are laminated in the width direction of the gasket. The transverse directional strength of the ePTFE is oriented in the vertical or z-axis direction of the gasket. Therefore, little strength is provided to the gasket in the radial direction. Therefore, gaskets taught in U.S. Patent Publication No. 2003/0003290 A1 may be prone to cold flow in the width direction and lack dimensional stability. Therefore, it is desirable that the plane of expansion of biaxially or multiaxially expanded PTFE gaskets be substantially parallel to the x-y plane of the gasket to provide dimensional stability and resistance to cold flow.
It would be desirable to provide a unitary, chemically resistant, dimensionally stable, high strength form-in-place gasket that can seal large diameter openings, especially glass-lined steel and FRP equipment flanges, upon the application of a relatively low compressive load. It is further desirable for a form-in-place gasket to be relatively flexible and can be installed using the common skive cutting techniques for overlapping the ends of the tape. Accordingly, several embodiments of an expanded PTFE form-in-place gasket material that overcome many of the limitations of the previous inventions are presented below.